To transition your betta to warmer water you will need to cup her and float the cup inside the heated tank and let the temperature stablize. You will also need to slowly adjust her to the new water by adding some of the tank water to the cup every 15 minutes or so. I recommend an adjustable heater... the least stressful way to do this is to only increase the temperature a few degrees every hour. Sometimes, it is best to only increase it one or two degrees in 24 hours.. Which is easier with an adjustable heater. It really depends on the fish, though. Some can handle quicker temperature changes better than others.
EDIT: The reason I personally think it would be better to turn on the heater before hand is so that you know it will keep a stable temp. Some of them are a few degrees off... If you set it at 78 degrees, it could heat the water a few degrees above or below that. However, I have personally also put the heater in after the fish was in the tank and have had no problems, but I didn't immediately set the heater to 78... I slowly raised it to 78. My younger betta did a lot better handling the temperature change than my older one did.
Also, live plants are better for your betta than artificial plants because they act as a filter, but they are harder to keep alive, obviously. There are a few hardy and easy to care for beginner aquatic plants. They use the fish waste as fertilizer and keep the tank cleaner. They are also part of the biological filter that would happen in nature. Have you read about cycling your tank? Are you planning on using a filter in the new tank?
Cycling works best in larger tanks of at least five gallons but it is not completely impossible to do in a 3.5 gallon tank. It also allows for less frequent water changes. Your betta will appreciate a larger tank as long as it is decorated enough, especially if she is a female. They tend to be more active than males with their shorter fins and love to have space to swim around.
Last edited by AyalaCookiejar; 12-30-2012 at 08:59 PM.